Many kinds of such apparatus are widely available commercially, and normally they use several such inductors for producing (and in some instances receiving) a plurality of magnetic fields which interact with the coins to be tested in different ways. The present invention concerns the structure of such inductors and their positioning within the apparatus, and reference is made to GB-A-1 452 740 and GB-A-2 094 008, for example, for further information as to how other aspects of such apparatus may be arranged and operated.
It is desirable, ideally, that the inductors having a particular function in each of the coin testing apparatuses manufactured to a given design should have identical operating characteristics. This is not achieved in practice due to tolerance variations in the actual construction of the inductors and in their assembly into the apparatuses.
Typically, an inductor comprises a core, frequently annular in shape, of high magnetic permeability material having a recess in its front face, the recess also being annular in the case of an annular core, and an inductive coil located in the recess.
U.S. Pat. No. 3,870,137 discloses (see its FIG. 6) such an inductor, in which the coil is on a former. The Applicants have regularly used such a design for many years, but there have been variations between the operating characteristics of different inductors. It has been found that this partly arises from the fact that, in assembling that type of inductor, it is usual for the former to be pressed or to fall until it contacts the bottom of the recess. Consequently any variations (as between one inductor and another) in the depth of the recess, or in the front-to-back dimension of the former, will result in related variations in the position of the front face of the coin relative to the front face of the core, and relative to the coin passageway side wall when the core is positioned against the side wall. These variations could affect the uniformity of performance between different inductors.